Measuring devices, in particular ultrasonic measuring devices, for warning a motor vehicle of the approach of an obstacle are conventional. An information signal is provided to the driver visually and/or acoustically as a function of distance. In particular, this renders it possible to prevent impact with obstacles that are not visible to the driver or for which the distance is difficult to estimate. However, if a parking space is too small to accommodate the vehicle, such a measuring device only provides the driver with limited assistance since the driver may unsuccessfully attempt to park the vehicle in the parking space. German Patent Application No. DE 196 16 447 describes a method for determining the length of a parking space and a parking assistance device suitable for the method via which the length of a parking space between a front and a back parked vehicle is determinable when driving slowly past. However, the described parking assistance device does not allow measurement of the distance to obstacles during the parking operation so that the driver is responsible for monitoring these distances during the parking operation.